By the red base in Hang 'Em, there is a dark grey wall connected to the wall. To get to it, jump on the little pillar that is part of the red base. Then throw a frag (or plasma) and jump as the nade detonates. You might make it, but it will probably take a few tries. So if there's over shield on the walkway, you should grab that, because the nade damage from nade jumping could leave you screwed.
Or you could have someone stand by the wall and jump on top of them, and then onto it. But I think you might need to throw a nade still. I prefer the nade jump, as it takes less time.